The present invention relates to a process for continuously casting a plate of polymethyl methacrylate. More particularly, it relates to an improved process for producing a cast plate of polymethyl methacrylate having an excellent thermal stability by continuous casting polymerization.
In general, a polymethyl methacrylate plate is superior in various properties, for example, flatness of surface, appearances such as transparency or color, mechanical properties, thermal properties, resistance to weathering and the like. Therefore, the plate is now in a wide use as a thermoplastic resin plate.
However, a polymethyl methacrylate plate produced by continuous casting polymerization of methyl methacrylate which has been developed in a recent year and attains a high productivity with a high speed is inferior in the thermal stability to a plate of polymethyl methacrylate produced by cell casting which has been widely adopted in commercial purpose, when exposed to relatively high temperatures (i.e. about 200.degree. C.), for example, on molding or fabricating. Thus, the polymer of methyl methacrylate once produced by continuous casting polymerization partially decomposes, and the resulting degradation monomers and low molecular compounds foam in the molded or fabricated products to impart a remarkable damage in the appearance of the products or to decrease the thermal and mechanical properties of the products. Therefore, a range of applicable temperatures for molding or fabricating the plate disadvantageously becomes narrow.
Various methods for preventing the said decomposition by heat have been proposed. One of the typical methods is the incorporation of a stabilizer such as a decomposition-inhibitor. As the stabilizer, amine compounds and phenol compounds are well known. However, these stabilizers can hardly be used in case of the polymer of methyl methacrylate having excellent optical properties, because the polymer incorporated with them is colored on heating and deteriorated in weatherability, transparency and color. Another typical method is the incorporation of sulfur-containing compounds as a thermal stabilizer [cf. Japanese Patent Publication Nos. 13383/1968 and 35977/1971], if necessary, with phenol compounds [cf. Japanese Patent Publication Nos. 1498/1968 and 7629/1972]. But, a satisfactory result has not yet been obtained.